Typically, high lift systems of commercial and military aircraft are powered by a centralized drive, also known as a power drive unit (PDU). Such drives are mounted in a central region of the fuselage and are controllable through a computerized control system or electronics unit. The PDU is coupled with a torque shaft system, also known as transmission shaft, which transfers mechanical power to geared actuators at flap or slat panel drive stations distributed along the trailing edge or leading edge of a wing. The control of the PDU is usually conducted by control computers, such as slat flap electronic control computers/unit (SFECU), which are commonly realized as a redundant arrangement of at least two independent SFECUs that are not only able to control but also to monitor the operation of the high lift system.
The PDU commonly comprises two independent motors that may be hydraulic or electric, which may be coupled with an output shaft by means of a speed summing differential gear. Each of the motors is commonly provided with a power off brake (POB) (typically an electromechanical brake) for arresting the motor in a commanded position. In some systems, while at least one of the two motors is commonly a hydraulic motor, the second motor may be realized as an electric motor, leading to a hybrid PDU. A wing tip brake, which is coupled with the transmission shaft and particularly placed in an outer region of the transmission shaft and/or in a tip region of the respective wing, is also capable of arresting and holding the transmission shaft. Each of the wing tip brakes are power off brake (POB) which arrest the system in an existing position.
Still further, high lift systems usually comprise torque limiters that are adapted for limiting the torque to be introduced into the transmission system. The torque limiters may be mechanical or electronic torque limiters, wherein the latter rely on constantly monitoring an introduced torque, taking authority over the motors of the PDU, and initiating limitation and/or a reversal once the torque exceeds a predetermined threshold. The torque limiters can be separate elements or integrated into the PDU.
High-lift systems often rely on specific brake engagement response times to mitigate failure scenarios such as un-commanded motion, asymmetry and flap/slat panel skew. In such failure scenarios if a monitored parameter of the system is found to be out of an acceptable range, the high-lift system annunciates the failure condition and commands the system brakes to engage preventing further motion. System parameters and functions such as threshold values, fault monitoring, fault persistence, brake electrical control circuits, and the brake itself all contribute to the overall response time of the brake to arrest the system. Moreover, brake systems can develop degraded performance over life due to electrical variations, mechanical wear and environmental exposure all of which will increase the brake's engagement response time. Degraded brake response times may prevent or degrade a high-lift system from mitigating a given failure scenario resulting in risk to the aircraft. As such, the need for improved actuator brake response time to arrest motion as a system without the need for ground test equipment would be a useful tool in mitigating certain failure modes.